Modified Vaccinia Ankara (MVA) virus is a member of the Orthopoxvirus family and has been generated by about 570 serial passages on chicken embryo fibroblasts of the Ankara strain of Vaccinia virus (CVA) (for review see Mayr, A., et al., Infection 3, 6-14 [1975]). As a consequence of these passages, the resulting MVA virus contains 31 kilobases less genomic information compared to CVA, and is highly host-cell restricted (Meyer, H. et al., J. Gen. Virol. 72, 1031-1038 [1991]). MVA is characterized by its extreme attenuation, namely, by a diminished virulence or infectious ability, but still holds an excellent immunogenicity. When tested in a variety of animal models, MVA was proven to be avirulent, even in immuno-suppressed individuals.
The Human Immunodeficiency virus (HIV) is the causative agent of the Acquired Immunodeficiency Syndrome (AIDS). Like all retroviruses, the HIV genome encodes the Gag, Pol and Env proteins. In addition, the HIV genome encodes further regulatory proteins, for example, Tat and Rev, as well as accessory proteins, such as Vpr, Vpx, Vpu, Vif and Nef.
Despite public health efforts to control the spread of the AIDS epidemic, the number of new infections is still increasing. The World Health Organization estimated the global epidemic at 37.8 million infected individuals at the end of the year 2003, and 36.1 million infected individuals at the end of the year 2000, 50% higher than what was predicted on the basis of the data about a decade ago (WHO & UNAIDS. UNAIDS, 2004). Without further improvements on comprehensive prevention mechanisms, the number of new HIV infections to occur, globally, this decade is projected to be 45 million (2004 Report on The Global AIDS Epidemic, UNAIDS and WHO).
Given the steady spread of the epidemic, a number of different HIV-1 vaccine delivery strategies, such as novel vectors or adjuvant systems, have now been developed and evaluated in different pre-clinical settings, as well as in clinical trials. The first vaccine candidate that entered a phase-III clinical trial is based on envelope gp 120 protein in alum formulations (Francis et al., AIDS Res. Hum. Retroviruses 14 (Suppl 3)(5): S325-31 [1998]). The results of the first clinical studies were discouraging.
The viral vaccines that were tested for efficacy in the past are usually based on single HIV proteins, such as Env. However, even if an immune response was generated against such a single protein, for example, Env, the immune response proved ineffective. One reason for the ineffectiveness is the high mutation rate of HIV, in particular with respect to the Env protein, reportedly resulting in viruses, in which the proteins are not recognized by the immune response induced by the vaccine. Since no effective prophylactic treatment is available, there is still a need to bring an effective vaccine to the clinic.
Several excellent properties of the MVA strain pertinent to its use in vaccine development have been demonstrated in extensive clinical trials (Mayr et al., Zbl. Bakt. Hyg. I, Abt. Org. B 167, 375-390 [1987]). During these studies, performed in over 120,000 humans, including high-risk patients, no side effects were seen (Stickl et al., Dtsch. med. Wschr. 99, 2386-2392 [1974]).
It has been further found that MVA is blocked in the late stage of the virus replication cycle in mammalian cells (Sutter, G. and Moss, B., Proc. Natl. Acad. Sci. USA 89, 10847-10851 [1992]). Accordingly, MVA fully replicates its DNA, synthesizes early, intermediate, and late gene products, but is not able to assemble mature infectious virions, which could be released from an infected cell. For this reason, namely, its replication-restricted nature, MVA serves as a gene expression vector.
More recently, MVA was used to generate recombinant vaccines, expressing antigenic sequences inserted either at the site of the thymidine-kinase (tk) gene (U.S. Pat. No. 5,185,146), or at the site of a naturally occurring deletion within the MVA genome (PCT/EP96/02926).
However, although the tk insertion locus is widely used for the generation of recombinant poxviruses, particularly for the generation of recombinant Vaccinia viruses (Mackett, et al. P.N.A.S. USA 79, 7415-7419 [1982]), use of this technology in MVA has several drawbacks. It was reported by Scheiflinger et al. that MVA is much more sensitive to modifications of the genome, when compared to other poxviruses, which can be used for the generation of recombinant poxviruses. Scheiflinger et al. reported, in particular, that one of the most commonly used sites for the integration of heterologous DNA into poxviral genomes, namely, the thymidine kinase (tk) gene locus, cannot be used to generate stable recombinant MVA. Any resulting tk(−) recombinant MVA proved to be highly unstable and, upon purification, immediately deleted the inserted DNA, together with parts of the genomic DNA of MVA (Scheiflinger et al., Arch Virol 141: pp 663-669 [1996]).
Instability and, thus, high probability of genomic recombination is a known problem within pox virology, and a drawback for vaccine production. Actually, MVA was established during long-term passages exploiting the fact that the viral genome of CVA is unstable when propagated in vitro in tissue cultured cells. Several thousands of nucleotides (31 kb) had been deleted in the MVA genome, which, therefore, is characterized by 6 major deletions, and numerous small deletions, in comparison to the original CVA genome.
The genomic organization of the MVA genome has been described recently (Antoine et al., Virology 244, 365-396 [1998]). The 178 kb genome of MVA is densely packed and comprises 193 individual open reading frames (ORFs), which code for proteins of at least 63 amino acids in length. In comparison with the highly infectious Variola virus, and also with the prototype of Vaccinia virus, namely the strain Copenhagen, the majority of ORFs of MVA are fragmented or truncated (Antoine et al., Virology 244, 365-396 [1998]). However, with very few exceptions, all ORFs, including the fragmented and truncated ORFs, get transcribed and translated into proteins. In the following description of the invention, the nomenclature of Antoine et al. (supra) is used and, where appropriate, the nomenclature based on HindIII restriction enzyme digest is also indicated.
To date, only the insertion of exogenous DNA into the naturally occurring deletion sites of the MVA genome reportedly led to stable recombinant MVAs (PCT/EP96/02926). Unfortunately, there are only a restricted number of naturally occurring deletion sites in the MVA genome. Thus, a need exists for the identification of additional stable insertion sites, particularly those that can be useful for generation of MVA-based vaccines for treatment and/or prevention of AIDS.